Daniel K. Lathrop

Nuclear magnetic resonance using a high temperature superconducting quantum interference device

In this letter, we present results of experiments in which nuclear magnetic resonance (NMR) signals were detected using an untuned pickup coil coupled to a high temperature superconductor (HTS) based superconducting quantum interference device (SQUID). We were able to operate the HTS SQUID in the presence of static fields of up to 500 Gauss and radio frequency fields of up to 6 Gauss. The ability of a HTS SQUID to detect NMR signals opens up the possibility of using a sensitive detector for practical applications involving NMR that require a broad detection bandwidth.

High performance superconducting quantum interference device feedback electronics

A dc superconducting quantum interference device (SQUID) modulation and feedback circuit operating at a modulation frequency of 16 MHz has been constructed. Using a novel wide band superconducting thin film transformer to impedance match the SQUID to a rf amplifier allows the system to operate at the SQUID noise level for most types of low‐TC SQUIDs. This system has a closed loop bandwidth exceeding 2.5 MHz and a slew rate greater than 1×106Φ0/s at frequencies up to 1 MHz. This greatly improved performance compared to existing modulation methods can be obtained without enhancing the transfer function of the SQUID. The system allows low‐ and high‐TC SQUID magnetometers and gradiometers to be operated totally unshielded without unlocking in the dc, 60 Hz, and radio frequency electromagnetic fields present in most SQUID applications.

Detection of TNT and RDX landmines by standoff nuclear quadrupole resonance

Nuclear Quadrupole Resonance (NQR) combines the compound specific detection capability offered by chemical offered by chemical detection techniques with the spatial coating capability and convenience of an induction coil metal detector. In this paper we present the first results of the detection of TNT by NQR with sufficient for detection of many antipersonnel mines and essentially all antitank mines. In addition, we review the result of a blind in-field demonstration of the system in detecting RDX in which 28 out of 31 RDX-only targets were found with 1 false alarm in a 110 m test lane, and a second test in which 21 out of 21 RDX mines were found with zero false alarms at a clearance rate of 1.1 m2 per minute.

Sensitive high-T/sub c/ SQUID magnetometers for unshielded operation

We have developed sensitive highly reliable high-T/sub c/ SQUID magnetometers a reproducible SNS junction fabrication process. In order to enable unshielded operation in the earths field, we have incorporated flux dams into direct-coupled SQUID magnetometers. By using up to four pickup coils in parallel, each with its own flux dam, an effective area of 0.57 mm/sup 2/ has been achieved on 24 mm/spl times/24 mm substrates using 100 /spl mu/m wide pickup coils. We have demonstrated a magnetic field noise sensitivity of 80 fT//spl radic/Hz 10 Hz and 77 K in an externally applied field 60 /spl mu/T. This magnetic field sensitivity unchanged from the zero-field measurement.

Land mine detection by nuclear quadrupole resonance

Nuclear Quadrupole Resonance (NQR) combines the compound specific detection capability offered by chemical detection techniques with the spatial localization capability and convenience of an induction coil metal detector. In the 16 years since NQR was last applied to mine detection in the U.S., there has been considerable improvement in the basic techniques. This paper reviews the progress achieved under a recent initiative to detect landmines by NQR. Two basic technical developments are summarized: the design of a detection coil suitable for probing the ground for landmines buried at typical depths, and an increase in the NQR signal obtained from the explosive TNT. In addition, we report the sensitivity of an NQR detection system to detect the electromagnetic response of metal-cased landmines.

Wideband front end for high-frequency SQUID electronics

We report the development of a wide-bandwidth, low-noise front end to facilitate the high frequency operation of both LTS and HTS DC SQUIDs. A transformer of novel design non-resonantly matches the dynamic impedance of the SQUID through a twisted pair to a room-temperature amplifier. The amplifier uses active feedback techniques to flatten the response of the signal path. When operated in conjunction with the high frequency SQUID feedback electronics (developed by IBM T.J. Watson Labs) with a modulation frequency of 16 MHz, and a Quantum Design DC SQUID, the system achieved a slew rate of 1.9/spl times/10/sup 6/ /spl Phi//sub 0/ s/sup -1/, a closed-loop performance of 5 MHz and a noise level of 5.5 /spl mu//spl Phi//sub 0//(Hz)/sup 1/2/. The response from SQUID input to amplifier output was relatively flat from 2 MHz to 25 MHz, with usable signal to 45 MHz. We believe the performance is limited by cabling and expect significant improvements in miniaturized design.

Advances in the engineering of quadrupole resonance landmine detection systems

Advances in the engineering of Quadrupole Resonance (QR) sensors for landmine detection have resulted in improved performance, as well as massive reductions in power, size and weight. The next generation of vehicle-mounted QR confirmation sensors is over an order of magnitude smaller and more power efficient than the system fielded in 2002 and 2003. Early prototypes have also demonstrated a significant improvement in TNT sensitivity, and similar improvements are anticipated in RDX sensitivity during Q1 2005. Blind test results from 2003 confirm the radio frequency interference and piezo-electric ringing immunity of the Quantum Magnetics QR Confirmation Sensor (QRCS).

Development of a quadrupole resonance confirmation system

Quantum Magnetics has developed a Quadrupole Resonance (QR) system for the detection of anti-tank and anti-vehicle landmines. The QR confirmation sensor (QRCS) is a part of the Army GSTAMIDS Block 1 program and is designed to confirm the presence of landmines initially flagged by a primary sensor system. The ultimate goal is to significantly reduce the number of sites that require neutralization or other time consuming investigation into the presence of a landmine. Government tests in 2002 and 2003 demonstrated the performance of the system in a wide variety of conditions including high radio frequency interference (RFI) and piezo electric ringing (PER) environments. Field test results are presented along with an overall description of the system design and methods used to solve prior issues with RFI and PER.

New amplifier configuration based on the circulating current state of a DC SQUID

DC and RF SQUIDs can be biased so that they switch between clockwise and counterclockwise circulating current states under the effect of internal noise and Josephson oscillations. Under this condition, the time averaged circulating current is very susceptible to small changes in flux bias, and for the DC SQUID, changes in current bias also. Signal gains of over 25 dB have been achieved in LTS SQUIDs, and 10 dB in HTS devices when applying the signal via flux or bias current modulation. The measured gains show excellent agreement with simulation and analytic expressions. There are several advantages with the new SQUID operating mode: applying the signal via the bias current line removes the need to make multi-turn superconducting input coils, and the resulting gain is frequency independent up to the GHz regime. The resulting amplifier has significant potential benefit for very low noise amplification up to 1 GHz, and ultra wideband readout of electrically short antennas.

Field test results of a nuclear quadrupole resonance land mine detection system

We report on field test results conducted during 1999 in Bosnia and at the Army Mine Training School, Fort Leonard Wood, MO, on a ne prototype landmine detection system. In all test, non-metallic, anti-personnel (AP) and anti-tank (AT) landmines were detected via the NQR explosive signature with a probability of detection of 100 percent. The initial false alarm rate for the AP mine test was < 5 percent and was reduced to zero by a subsequent remeasurement. The test included typical burial depths and a variety of ground and weather conditions. In addition, the system can tolerate very high levels of metallic clutter and has repeatedly achieved zero false alarm rate when scanning for buried explosives at an EOD test range.